Wireless electronic communications encounter particular difficulties in certain types of environments or situations. In urban environments, reflections and multi-path are problematic. In underwater or underground environments, signal attenuation presents a particular problem for RF signals. In military applications, signal interception and signal jamming are significant concerns with RF communications.
Accordingly, wireless communications systems have been developed that relay upon magneto-inductive technology. Magneto-inductive communications use quasi-static low frequency, AC magnetic fields. A quasi-static magnetic field differs from an electromagnetic field in that the electric field component is negligibly small. A quasi-static magnetic field does not propagate as an electromagnetic wave, but instead arises through induction. Accordingly, a quasi-static magnetic field is not subject to the same problems of reflection, refraction or scattering that radio frequency electromagnetic waves suffer from, and may thus communicate through various media (e.g. earth, air, water, ice, etc.) or medium boundaries.
Typical magneto-inductive (MI) systems include a magneto-inductive transmitter and a magneto-inductive receiver, and operate in the range of a few hundred Hz to 10 kHz. More typically, the operating frequency of an MI system is in the range of 500 to 3000 Hz.
MI systems find application in undersea operations, mining, military, and other such fields. By way of example, U.S. Pat. No. 6,253,679 to Woodall et al. describes a specific magneto-inductive remote triggering system for line charges used in amphibious assaults. MI systems may also be used for other wireless communications purposes, including, in some cases, the transmission voice data for person to person communications.
A practical limitation of MI systems is that the range of MI transmissions is relatively short compared with RF systems. Some MI systems, with sufficient power and sufficiently large antennas, may achieve effective ranges of several kilometers; however, many MI systems have a range of a few hundred meters. There are circumstances in which it would be advantageous to communicate the coded signals for an MI system over a longer range.